Re: [PATCH 0/4] arm64: Support the TSO memory model

[Eric Curtin]

On Sat, 20 Apr 2024 at 13:13, Eric Curtin <ecurtin@xxxxxxxxxx> wrote:
>
> On Fri, 19 Apr 2024 at 18:08, Will Deacon <will@xxxxxxxxxx> wrote:
> >
> > On Thu, Apr 11, 2024 at 11:19:13PM +0900, Hector Martin wrote:
> > > On 2024/04/11 22:28, Will Deacon wrote:
> > > >   * Some binaries in a distribution exhibit instability which goes away
> > > >     in TSO mode, so a taskset-like program is used to run them with TSO
> > > >     enabled.
> > >
> > > Since the flag is cleared on execve, this third one isn't generally
> > > possible as far as I know.
> >
> > Ah ok, I'd missed that. Thanks.
> >
> > > > In all these cases, we end up with native arm64 applications that will
> > > > either fail to load or will crash in subtle ways on CPUs without the TSO
> > > > feature. Assuming that the application cannot be fixed, a better
> > > > approach would be to recompile using stronger instructions (e.g.
> > > > LDAR/STLR) so that at least the resulting binary is portable. Now, it's
> > > > true that some existing CPUs are TSO by design (this is a perfectly
> > > > valid implementation of the arm64 memory model), but I think there's a
> > > > big difference between quietly providing more ordering guarantees than
> > > > software may be relying on and providing a mechanism to discover,
> > > > request and ultimately rely upon the stronger behaviour.
> > >
> > > The problem is "just" using stronger instructions is much more
> > > expensive, as emulators have demonstrated. If TSO didn't serve a
> > > practical purpose I wouldn't be submitting this, but it does. This is
> > > basically non-negotiable for x86 emulation; if this is rejected
> > > upstream, it will forever live as a downstream patch used by the entire
> > > gaming-on-Mac-Linux ecosystem (and this is an ecosystem we are very
> > > explicitly targeting, given our efforts with microVMs for 4K page size
> > > support and the upcoming Vulkan drivers).
> >
> > These microVMs sound quite interesting. What exactly are they? Are you
> > running them under KVM?
>
> It's the magic of libkrun. This is one of the git repos in the family
> of libkrun, it has a wide array of use cases, which I personally won't
> do much justice explaining all then, this is just one
> repo/tool/usecases:
>
> https://github.com/containers/krunvm
>
> https://sinrega.org/running-microvms-on-m1/

Sorry for the double post, meant to share this one for the Asahi
emulator usecase. Sergio's blogs are great in general:

https://sinrega.org/2023-10-06-using-microvms-for-gaming-on-fedora-asahi/

Is mise le meas/Regards,

Eric Curtin

>
> CC'ing @Sergio Lopez Pascual the lead of krun in general.
>
> Is mise le meas/Regards,
>
> Eric Curtin
>
> >
> > Ignoring the mechanism for the time being, would it solve your problem
> > if you were able to run specific microVMs in TSO mode, or do you *really*
> > need the VM to have finer-grained control than that? If the whole VM is
> > running in TSO mode, then my concerns largely disappear, as that's
> > indistinguishable from running on a hardware implementation that happens
> > to be TSO.
> >
> > > That said, I have a pragmatic proposal here. The "fixed TSO" part of the
> > > implementation should be harmless, since those CPUs would correctly run
> > > poorly-written applications anyway so the API is moot. That leaves Apple
> > > Silicon. Our native kernels are and likely always will be 16K page size,
> > > due to a bunch of pain around 16K-only IOMMUs (4K kernels do boot
> > > natively but with very broken functionality including no GPU
> > > acceleration) plus performance differences that favor 16K. How about we
> > > gate the TSO functionality to only be supported on 4K kernel builds?
> > > This would make them only work in 4K VMs on Asahi Linux. We are very
> > > explicitly discouraging people from trying to use the microVMs to work
> > > around page size problems (which they can already do, another
> > > fragmentation problem, anyway); any application which requires the 4K VM
> > > to run that isn't an emulator is already clearly broken and advertising
> > > that fact openly. So, adding TSO to this should be only a marginal risk
> > > of further fragmentation, and it wouldn't allow apps to "sneakily" "just
> > > work" on Apple machines by abusing TSO.
> >
> > I appreciate that you're trying to be constructive here, but I don't think
> > we should tie this to the page size. It's an artifical limitation and I
> > don't think it really addresses the underlying concerns that I have.
> >
> > Will
> >